Add multi-server data model + legacy DB migration (step 0)

Adds Server.ID/Name, ServerInterface.Name, Client.ServerID, and a new
ServerSetting type as additive fields so existing single-server code
paths keep working unchanged. jsondb.Init() now detects a pre-existing
single-server db/server/ layout and mirrors it into new servers/,
server_settings/, server_hashes/ collections plus backfills ServerID
on existing clients, without touching/removing the legacy files yet.

model/client.go

@@ -7,6 +7,7 @@ import (
 // Client model
 type Client struct {
 	ID              string    `json:"id"`
+	ServerID        string    `json:"server_id,omitempty"`
 	PrivateKey      string    `json:"private_key"`
 	PublicKey       string    `json:"public_key"`
 	PresharedKey    string    `json:"preshared_key"`